The service life of insulated power cables is known to be shortened by a prebreakdown phenomenon termed water-treeing. Consequently, insulation based on polymeric compositions, as for example, compositions of ethylene polymers, has been modified by the addition thereto of various additives for the purpose of providing insulation characterized by resistance to water-treeing. Water-trees occur when insulation is subjected to an electrical field, over a prolonged period of time, while in an environment in which water is present. Water-trees are so named because of a resemblance to branched trees. In actuality, water-trees are fine passages or voids which are visibly white when filled with water but become invisible when dry. For purposes of observation, specimens with water-trees must be boiled in water or dyed.
Water-trees start to grow at points of voltage stress in the insulation. Points of stress might be at sites in the insulation at which there are voids, contamination, or irregularities, for instance at the interface between the insulation and a semi-conductive layer. In the case of voids of contamination, trees tend to form at opposite sides of the point of stress (the nucleus) and, in time, grow to resemble a bow-tie. Trees initiated at an interface grow in one direction in the form of a bush and are referred to as vented trees. The two types of water-trees, bow-tie and vented, appear to start and to grow by different mechanisms, as indicated by their differing response to many of the water-tree retardants that have been examined.
Up until the present time, no single water-tree retardant system has been developed which, when added to a polymeric composition, results in a composition which has a commercially acceptable balance of properties including resistance, over prolonged periods of time, to the formation of bow-tie trees and vented water-trees.